Video enhancement techniques are used to make video appear sharper. One type of video enhancement technique involves enhancing edges or high frequency elements of the video. Conventional edge enhancement uses a high pass filter (HPF) or band pass filter (BPF) and an adder with filtering gain control. The conventional technique for enhancing edges is based on the Mach band effect. Edge enhancement is achieved by increasing the simultaneous contrast of the edge. The variation of the simultaneous contrast needs to be clamped into a limited range to avoid undershoot or overshoot (i.e., for an 8 bit unsigned quantization system, the limits are 0 and 255).
Referring to FIG. 1, a graph 10 is shown illustrating an example of over emphasized artifacts that can occur with the conventional enhancement technique. An input signal, captured from a monochrome image with a quantization range of zero to ten, can have a black region having a range from zero to less than two, a visible (or perceivable) region having a range from two to eight and a white region having a range from greater than eight to ten. From a perceptual view, the pixels located in the black region or the white region are not distinguishable.
An enhanced output signal (i.e., mixed original and a gain controlled output of a HPF or BPF) can have overshoot and undershoot conditions (i.e., pixel values greater than 10 or less than 0, respectively). The overshoot and undershoot conditions can be corrected by clamping circuitry. However, some pixels that were in the black region or the white region in the input signal (i.e., pixels 2, 4, 16 and 18) can be shifted into the visible region (i.e., the range from two to eight) after enhancement. The pixels with values shifted into the visible range cause artifacts (i.e., noise in the black and white regions). In general terms, the enhancement can cause white flakes in the black region and grey flakes in the white region.
To reduce the over emphasized artifacts a filtering gain control scheme is used. However, the artifacts will be highlighted if the gain control is not set properly. Unfortunately, a proper setting of the filtering gain is very difficult because the HPF or the BPF operates based on the frequency component of the video and ignores the amplitude component of the video.
Referring to FIGS. 2(A-C), graphs are shown illustrating characteristics of a high pass filter. Input signals presented to a high pass filter can have different DC (direct current) values (illustrated in FIG. 2A). However, the output of the high pass filter is related to frequency response and phase relationship and independent of the DC value of the input (as illustrated in FIG. 2B). The outputs of the high pass filter can be identical for the input signals with different DC values (FIG. 2C).
The conventional techniques have disadvantages of emphasizing pixels that human vision can not distinguish and causing artifacts. The filtering gain adjustment is based on frequency domain response (filter output) only and ignores the time domain characteristic of the input signal (i.e., the DC value).
It would be desirable to have a method and/or apparatus for adaptively controlling the filtering gain to avoid artifacts.